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| United States Patent |
5,101,272
|
|
Plut
, et al.
|
March 31, 1992
|
Dual bandwidth/gain video preamplifier
Abstract
A TV camera (22) generates a low level video signal which represents a
sensed image pattern from an image intensifier (18). A preamplifier
circuit (24) boosts the low level video signal. The preamplifier is
selectively switched between feedback paths (74,76) to select the gain of
the preamplifier, and between various filter paths (84,88) to select the
bandwidth. Various display processing components such as a digital
acquisition system (42) and video tape or video disc recorder (44) are
provided to process video signals from the preamplifier circuit. A monitor
(26) containing circuitry to generate synchronization signals, to adjust
brightness, to adjust images aspect ratio, and to adjust image resolution
is provided to convert the video signals into a man readable display. A
video switch (40) selectively routes the video signals from the
preamplifier to one of the display processing components and from these
components to the monitor. The route, the gain, and the bandwidth are
controlled by input from a console (30).
| Inventors:
|
Plut; Leonard F. (Concord Township, OH);
Vagi; Robert J. (Broadview Heights, OH)
|
| Assignee:
|
Picker International, Inc. (Highland Hts., OH)
|
| Appl. No.:
|
392479 |
| Filed:
|
August 11, 1989 |
| Current U.S. Class: |
378/98.2; 378/98.5 |
| Intern'l Class: |
H04N 007/01; H04N 007/12; H04N 005/268 |
| Field of Search: |
378/99
358/184,140,141,11,137,174,181,136
|
References Cited
U.S. Patent Documents
| 3567854 | Mar., 1971 | Tschantz et al. | 358/11.
|
| 4204225 | May., 1980 | Mistretta | 378/99.
|
| 4303940 | Dec., 1981 | Ciciora | 358/140.
|
| 4527197 | Jul., 1985 | Nolte | 358/168.
|
| 4543605 | Sep., 1985 | Verhoeven | 378/99.
|
| 4573183 | Feb., 1986 | Relihun | 378/99.
|
| 4574279 | Mar., 1986 | Roberts | 358/140.
|
| 4581635 | Apr., 1986 | Franke | 378/99.
|
| 4665427 | May., 1987 | Beckley et al. | 358/11.
|
| 4719644 | Jan., 1988 | Herzog et al. | 358/140.
|
| 4739390 | Apr., 1988 | Achiha et al. | 358/11.
|
| 4881124 | Nov., 1989 | Yokouchi et al. | 378/99.
|
| 4910592 | Mar., 1990 | Shroy, Jr. et al. | 358/174.
|
| 4922916 | May., 1990 | Ermert et al. | 378/99.
|
| 4924487 | May., 1990 | Nishiki | 378/99.
|
| 4930144 | May., 1990 | Plut et al. | 378/99.
|
| 4939577 | Jul., 1990 | Schreurs | 358/140.
|
| Foreign Patent Documents |
| 8600483 | Jan., 1986 | JP | 358/168.
|
Primary Examiner: Kostak; Victor R.
Assistant Examiner: Murrell; Jeffrey S.
Attorney, Agent or Firm: Fay, Sharpe, Beall, Fagan, Minnich & McKee
Parent Case Text
This application is a continuation in part of application Ser. No. 203,510
filed May 25, 1988 which is a continuation of application Ser. No.
936,470, filed Nov. 25, 1986.
Claims
Having thus described the preferred embodiment, the invention is now
claimed to be:
1. A multi-rate video system comprising:
a camera for generating low level video signals representing a sensed image
pattern;
a preamplifier circuit for boosting the low level video signals;
a gain selecting means including:
a plurality of feedback resistors;
a feedback resistor switching means for selectively connecting at least one
of the feedback resistors into a feedback path;
a bandwidth selecting means for selecting a bandwidth of signals from the
preamplifier circuit;
a video recorder for selectively recording and replaying video signals from
the preamplifier circuit;
a data acquisition means for selectively processing the video signals from
the preamplifier circuit;
a monitor means for converting the video signals into a man-readable
display;
a video switch means for selectively routing the video signals from the
preamplifier circuit to a selected one of the monitor means, the recorder,
and the data acquisition means and from a selected one of the preamplifier
circuit, the recorder, and the data acquisition means to the monitor
means;
a console for selectively controlling the video switch means, the gain
selecting means, and the bandwidth selecting means.
2. A multi-rate video system comprising:
a camera for generating low level video signals representing a sensed image
pattern;
a preamplifier circuit for boosting the low level video signals;
a gain selecting means for selecting a gain of the preamplifier circuit;
a bandwidth selecting means including:
a plurality of filter paths;
a filter switching means for selectively connecting at least one of the
filter paths with an output of the preamplifier circuit whereby a
bandwidth of the video signals leaving the preamplifier circuit is
selectable;
a video recorder for selectively recording and replaying video signals from
the preamplifier circuit;
a data acquisition means for selectively processing the video signals from
the preamplifier circuit;
a monitor means for converting the video signals into a man-readable
display;
a video switch means for selectively routing the video signals from the
preamplifier circuit to a selected one of the monitor means, the recorder,
and the data acquisition means and from a selected one of the preamplifier
circuit, the recorder, and the data acquisition means to the monitor
means;
a console for selectively controlling the video switch means, the gain
selecting means, and the bandwidth selecting means.
3. A selectable, multiple gain/multiple bandwidth preamplifier system
comprising:
a preamplifier input that is adapted to be connected with a current source;
at least one amplification stage operatively connected with the
preamplifier input;
a plurality of feedback resistors;
a feedback resistor switching means for selectively connecting the feedback
resistors into a feedback path around the amplification stage to select
gain;
a plurality of filter paths, each filter path passing a selected bandwidth;
and
a filter switching means for selectively connecting the filter paths with
an amplification stage to select bandwidth.
4. The preamplifier system of claim 3 wherein the plurality of feedback
resistors are variable resistors.
5. The preamplifier system of claim 3 wherein the plurality of feedback
resistors include at least a first and a second feedback resistor, the
second feedback resistor being four times the first.
6. The preamplifier system of claim 3 wherein the filter switching means
switches between a first filter path which passes a bandwidth of generally
5 MHz and a second path that passes a bandwidth of generally 20 MHz.
7. The preamplifier system of claim 3 wherein a first of the filter paths
has a decreased bandwidth relative to a second of the filter paths such
that switching to the first filter path results in a decrease in system
bandwidth and in a proportional decrease in system noise.
8. A diagnostic imaging system comprising:
a camera means for producing low level video signals representing a
diagnostic image, the camera means having a plurality of preselected
operating modes;
a filtering means for selectively filtering the video signals with one of a
plurality of preselected filter functions, each filter function
corresponding to one of the preselected operating modes;
a monitor means for converting the video signals into a man-readable
representation of the diagnostic image in a raster format which has one of
a preselected number of lines, each preselected number of lines
corresponding to one of the preselected operating modes; and,
an adjusting means for adjusting a brightness of said man-readable
representation produced by said monitor means; and
a selecting means for selecting one of the preselected operating modes, the
selecting means, the filtering means, the monitor means, and the adjusting
means being operatively connected such that the filtering means selects
the filter function in direct response to the preselected number of raster
lines which are used with the selected operating mode, and the adjusting
means adjusts the brightness of the man-readable representation in
accordance with the selected operating mode.
9. A diagnostic imaging system comprising:
a camera means operable in at least first and second modes for generating a
video signal with a first number of raster lines per image in the first
mode, and a second number of raster lines per image in the second mode;
a preamplifier means for amplifying the video signal from the camera means
with a preamplifier gain, the preamplifier means amplifying video signals
with frequency components over a preamplifier bandwidth;
a changing means for changing the preamplifier gain in response to changing
from the first mode to the second mode, the changing means including:
a least first and second feedback paths; and,
a feedback switching means for switching between the first and second
feedback paths in response to changing between the first and second modes.
10. A diagnostic imaging system comprising:
a camera means operable in at least first and second modes for generating a
video signal with a first number of raster lines per image in the first
mode, and a second number of raster lines per image in the second mode;
a preamplifier means for amplifying the video signal from the camera means
with a preamplifier gain, the preamplifier means amplifying video signals
with frequency components over a preamplifier bandwidth;
a changing means for changing at least the preamplifier bandwidth in
response to changing from the first mode to the second mode, the changing
means including:
at least first and second filter paths which pass first and second
bandwidths, respectively;
a filter switching means for switching between the first and second filter
paths in response to changing between the first and second modes.
11. The diagnostic imaging system as set forth in claim 10 wherein the
changing means further includes:
at least first and second feedback paths; and,
a feedback switching means for switching between the first and second
feedback paths in response to changing between the first and second modes.
12. The diagnostic imaging system as set forth in claim 11 wherein the
first and second feedback paths include first and second feedback
resistors, respectively, a ratio of a resistance of the second feedback
resistor to a resistance of the first feedback resistor is substantially
the same as a ratio of the first number of raster lines to the second
number of raster lines.
13. The diagnostic imaging system as set forth in claim 12 wherein a ratio
of the first bandwidth to the second bandwidth is substantially the same
as the ratio of the first number of raster lines to the second number of
raster lines.
14. The diagnostic imaging system as set forth in claim 10 wherein a ratio
of the first bandwidth to the second bandwidth is substantially the same
as a ratio of the first number of raster lines to the second number of
raster lines.
15. A method of diagnostic imaging comprising:
converting patterns of radiation to video signals which represent the
patterns;
selecting a number of raster lines per image of the video signals;
amplifying the video signals with a selected gain;
selectively passing the video signals to one of first and second filter
paths which limit the video signal to one of first and second bandwidths,
respectively, in accordance with selected number of raster lines per
image;
converting the video signals to a man-readable display of the radiation
pattern.
16. A method of diagnostic imaging comprising:
converting patterns of radiation to video signals which represent the
patterns;
selecting a number of raster lines per image of the video signals;
amplifying the video signals with a selected gain;
feeding back a portion of the amplified video signals through one of first
and second feedback paths to adjust the selected gain, in accordance with
selected number of raster lines per image;
limiting the video signals to a selected bandwidth;
converting the video signals to a man-readable display of the radiation
pattern.
17. A method of diagnostic imaging comprising:
converting patterns of radiation to video signals which represent the
patterns;
selecting between a first number of raster lines and a second number of
raster lines per image of the video signals;
amplifying the video signals with a selected gain;
limiting the video signals to a selected bandwidth;
changing the gain and bandwidth in accordance with a ratio of the first and
second numbers of raster lines;
converting the video signals to a man-readable display of the radiation
pattern.
Description
BACKGROUND OF THE INVENTION
The present invention relates to the art of amplifying video signals. The
invention finds particular application in conjunction with
fluoroscopic/radiographic medical imaging systems and will be described
with particular reference thereto. However, it is to be appreciated that
the invention will also find application in multi-rate scan video camera
systems with remotely selectable dual bandwidth/gain video preamplifiers.
In an x-ray fluoroscopic/radiographic imaging system, a radiation source
directs x-rays through a patient onto an input surface of an image
intensifier tube. The image intensifier tube converts a relatively large
area of x-rays into a relatively small brightened visual image which
corresponds to the x-ray pattern emerging from the patient. The image from
the image intensifier tube is converted to a video signal by a video
camera.
In the fluoroscopic/radiographic imaging system, the video signals from the
video camera can be used in a variety of manners. First, the video signals
can be sent directly to a viewing monitor which produces an image. In
another option, the video signals can be directed to a variety of display
processing components including digital acquisition systems, video tape
recorders, and video disc recorders. These components digitize, enhance,
and store the video signals so that they may be played back on a monitor
at a later time.
One feature of these multi-mode diagnostic imaging systems is the different
parameters which must be accommodated in changing modes. For example, a
standard fluoroscopy mode employs a 525 lines per screen image. Whereas a
high resolution radiographic mode employs a 1049 lines per screen image.
Numerous adjustments, such as brightness, must be made for visual
compatibility as a function of which line rate is selected. In playing
back the stored information, the aspect of the monitor produced image is
typically changed from a 1:1 to a 4:3 aspect ratio. Depending upon the
mode selection, various degrees of resolution are obtained. These
differing parameters are accommodated by adjustments to the monitor to
produce an acceptable video image. In the past, one manner to satisfy
these varying requirements was to create a system containing multiple
monitors, each being preset for the operation in accordance with the
various combination of parameters necessary for each operating mode. The
parent applications disclose a system that employs a single monitor with
associated control circuitry capable of displaying images from the various
operating modes.
One of the problems encountered when forcing a plurality of modes into a
common display format, is that some modes have a more unfavorable signal
to noise ratio in the display format. Specifically, in the medical field,
the video camera tube is commonly operated in three modes; (1) an
interlace mode with 60 frames per second, (2) a non-interlaced progressive
mode operating at 30 frames per second, and (3) a non-interlaced
progressive mode at 7.5 frames per second. The slower third mode normally
has significantly more image deterioration than the first two modes.
The present invention contemplates a new and improved camera system and
method which over comes the above referenced image deterioration problems
and others.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a multi-rate video
system is provided. A camera generates low level video signals which
represent a sensed image pattern. A preamplifier circuit, which includes a
gain selecting means and a bandwidth selecting means, boosts the low level
video signals. A video recorder selectively records and replays video
signals from the preamplifier circuit and a data acquisition means
selectively processes video signals from the preamplifier circuit. A
monitor containing circuitry to generate synchronization signals, to
adjust brightness, to adjust aspect ratio, and to adjust image resolution
is provided. The monitor is used to convert the video signals into a man
readable display. A video switch selectively routes the video signals from
the preamplifier circuit to the monitor, the recorder or the data
acquisition means and from the recorder or data acquisition means to the
monitor. A console is provided for selectively controlling the video
switch, the gain selecting means and the bandwidth selecting means.
In accordance with another aspect of the present invention, a diagnostic
imaging system is provided having a source of radiation. A multi-mode
detecting means detects radiation patterns. A selecting means selects one
of the operating modes of the multi-mode detecting means. A preamplifier
circuit selectively amplifies the low level video signals in accordance
with the operating mode selected by the selecting means. A monitor
receives the video signals and produces a man-readable image of the
radiation pattern represented by the video signals. The image is in a
raster format where the operating mode is selected in accordance with the
number of raster lines in the raster format. An adjusting means adjusts
the brightness of the image produced by the monitor as a function of the
number of lines of the display image.
One advantage of the present invention is that it improves the
signal-to-noise ratio.
Another advantage of the present invention is an improved dynamic range for
the post amplifiers. The video gain for amplifiers after the preamplifier
is unaltered.
Yet another advantage of the present invention is that it provides for a
remotely selectable dual bandwidth/dual gain video preamplifier.
Still further advantages will be apparent to those of ordinary skill in the
art upon reading and understanding the following detailed description of
the preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be embodied in various steps and arrangements of steps
and in various components and arrangements of components. The drawings are
only for purposes of illustrating a preferred embodiment and are not to be
construed as limiting the invention.
FIG. 1 is a block diagram illustrating image acquisition, preamplification,
storage and monitoring components of a diagnostic imaging system in
accordance with the present invention;
FIG. 2 is a schematic diagram of portions of the camera and selectable dual
gain/dual bandwidth video preamplifier of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIG. 1, a diagnostic imaging system incorporating the
present invention is presented in generalized form. The system includes a
source of penetrating radiation 10 directed along a path 12. The path
causes the radiation source to pass through a patient 14 and to impinge
upon an input face 16 of an image intensifier tube 18. The x-rays from the
source which have passed through the patient 14 emerge and impinge upon
the intensifier tube's input face 16 in a unique radiation pattern. The
image tube 18 converts the relatively large area of x-rays into a smaller,
relatively bright visual image corresponding to the x-ray pattern which
emerged from the patient, at the image tube's output face 20.
A television camera 22 receives light output from the output face 20 of
image tube 18. The camera 22 produces analog video signals, including
synchronization information, which represent the viewed image. The video
signals, which are low level signals, are processed in a preamplifier
means or circuit 24 which boosts the low level video signals up to more
readily usable values and selectively filters the amplified signals
removing unused bandwidths.
The video and synchronization signals can be transmitted directly to a
monitor 26 for display. Alternatively, a video signal processing subsystem
28 may enhance or process the signals before they are sent to the monitor
26.
A console 30 having an attached keyboard 32 controls and selects whether
the video signal is to be transmitted directly to the monitor 26 from
camera 22 or is to be transmitted to one of the other processing or memory
sections located in the video signal processing subsystem 28 and
thereafter sent to monitor 26.
Looking in greater detail to the video processing subsystem 28, the video
signal outputted from the television camera 22 and the preamplification
circuitry 24 is transmitted to an input of a multi-input, multi-output
video switch unit 40. Other inputs of the video switch unit 40 are
connected to the outputs of a digital acquisition system 42 and a video
recorder 44. Outputs of video switch unit 40 are connected to the inputs
of the digital acquisition system 42, the video recorder unit 44, and the
monitor 26. The video switch unit 40 conveys both video signals and
control signals to the monitor 26.
The video switch unit 40 allows any of its inputs to contact any of its
outputs without video termination problems. The selection of the video
path through the switch unit 40 depends on which of the modes of operation
is selected for the system.
The console 30 interfaces the video processing subsystem 28 by a
multi-conductor cable which is interfaced to a programmable switch logic
circuit 46 by way of a system signal distribution circuit 48. The output
from the programmable switch logic circuit 46 is inputted into the
preamplifier circuit 24 to select its gain and bandwidth in accordance
with the operating mode chosen at console 30. In an alternative to this
method, a separate programmable switching unit (not shown) can be
associated with the preamplification circuitry 24 and signals from the
distribution circuitry 48 can be fed directly into this separate
programmable switch.
With reference to FIG. 2, light rays 50 in the form of the image pattern of
the intensifier tube 18 impinge on a target 54 of the camera 22. The
target 54 generally consists of a transparent signal electrode, such as a
glass plate 56, located on the front face of the tube and a thin layer of
photoconductive material 58 applied at the rear of the glass plate 56. The
photoconductive material 58 serves two main purposes. It is a light
sensitive element, and it forms the storage surface for the electrical
charge pattern that corresponds to the light image from the intensifier
tube 18, falling on the glass plate 56. The photoconductive material 58
has a large resistance when no light is impinging on its surface. Light
falling on the conductive material excites electrons into a conducting
state, thereby lowering the resistance of the photoconductive material at
the point of illumination.
A positive voltage 60 is applied to one side of the photoconductive layer.
On the other side, the scanning beam 62 scans the backside of the target
54. In the interval between successive scans of a particular spot, the
light rays 50 lower the resistance of the photoconductive material 58 in
relation to its intensity. Current then flows through the surface at this
point, and the back surface builds up a positive voltage until the beam
returns to scan the point again. The signal output current is generated
when the beam deposits electrons on these positively charged areas. An
equal number of electrons flow out of the signal electrode and through a
load resistor 64 which converts the current output video signal to a
voltage output video signal Vo. A corresponding output current i.sub.0 is
fed directly to a preamplification circuit 24. When the photoconductive
material is dark and its resistance is high, proportionally more current
flows to the preamplifier.
Applying light to the photoconductive material reduces the resistance in
that area thereby discharging the voltage which has built up on capacitor
elements 66. Thereafter, when the scanning beam senses a low voltage,
current will flow through the scanning beam to recharge the capacitor to
applied voltage rather than to the preamplifier.
The current i.sub.0 outputted to the preamplifier circuitry 24 is defined
by the relationship i.sub.0 =C de/dt which can be simplified to algebra
as: i.sub.0 =Ce/.DELTA.t, where
i.sub.0 =current out to preamplifier system;
C=the lump capacitance of the entire conductive target;
e=the voltage across the capacitor; and,
.DELTA.t=the time to scan the target area.
As previously noted, in the preferred diagnostic imaging system, and in
many other uses in the medical field, three types of scanning procedures
are used: (1) an interlaced mode with 60 fields per second, (2) a
non-interlaced progressive mode with 30 frames per second, and (3) a
progressive mode with 7.5 frames per second.
The first two procedures cause no change in time during scanning. Both
operate at the standard 30 frames per second. The third, 7.5 frames per
second, which is used in digital radiography, does change the time per
scan. Specifically, such a change alters the .DELTA.t from the above
equation. Under the conventional 7.5 frames per second procedure, only a
quarter of the signal current for a given light condition is passed to the
preamplifier circuitry 24, thereby, deteriorating the signal-to-noise
ratio by a factor of 4.
All real components, including those of the present preamplifier, have
thermal noise which is represented by:
v.sub.n =.sqroot.4KtR.DELTA.f
where
v.sub.n =rms noise voltage
K=Boltzmann's constant (1.37.times.10.sup.-23)
t=Absolute temperature, (.degree.Kelvin),
.DELTA.f=Noise bandwidth, (Hz),
R=Resistance (ohms).
This equation reduces at room temperature to:
v.sub.n =.sqroot.KR.DELTA.f
Note that the signal-to-noise ratio of a system is improved as R increases.
A trans-impedance amplifier 72 generates a voltage proportional to the
input current i.sub.0. The input current is amplified and a voltage is
produced of an appropriate amplitude for producing video images of the
subject 14.
A first feedback resistance 74 and a second feedback resistance 76, both of
which can be variable resistors, are selectively switched into the circuit
by a feedback resistance switch 78. When the system is switched to the
radiographic mode, 7.5 frames per second, the feedback switch 78 puts the
second feedback resistances 76 into the feedback loop. The second feedback
resistance is four (4), times the first feedback resistance 74--the same
ratio as the 7.5 frames per second mode to the 30 frames per second mode.
In the preferred embodiment, 1 megohm and 4 megohm feedback paths are
provided. Quadrupling the resistance quadruples the gain and voltage
output, thereby making it equivalent to the voltage output of the 30
frames per second mode. The additional resistance also increases the noise
by approximately the square root of 4, i.e. a factor of 2. Because the
signal increased is by 4 and the noise is increased by only 2, the
signal-to-noise ratio is approximately doubled.
To accommodate camera system scan requirements in terms of vertical frame
times, overall system bandwidth and gain is regulated to a maximum of 2
microamps at 20 MHz for a system operating at 30 frames per second and 0.5
microamps at 5 MHz for a system operating at 7.5 frames per second.
The present invention further improves the signal to noise ratio by
recognizing that noise is uniformly distributed throughout the frequency
range. The preamplifier has a 20 MHz bandwidth to accommodate the 30
frames per second signal. But the 7.5 frames per second signal only takes
advantage of 5 MHz of the available bandwidth. The present invention
improves the signal-to-noise ratio with a bandwidth selecting means 80.
Specifically, when the operating mode using 7.5 frames per second is
selected, a second selector switch 82 switches in the 5 MHz band pass
filter 84 which filters a signal received from a voltage buffer amplifier
section 86 of the preamplifier 24, so that only the 5 MHz bandwidth is
passed. This eliminates the unused three-fourths of the 20 MHz bandwidth
hence three-fourths of the noise.
When operating in the 30 frames per second mode, the switch 82 selects a
path 88 that allows 20 mHz to pass. When the bandwidth of the preamplifier
components is selected at 20 mHz, the 20 mHz path may be a conductor. When
the noise generated in the preamplifier is equivalent to 45 db at 20 mHz
and keeping in mind that the contribution due to increased thermal noise
of the 4 megohm resistor is small compared to the noise of the input of
the first amplifier stage, the following improvements may be noticed.
__________________________________________________________________________
5 MHZ CASE
20 MHZ CASE w/4 megohm
DESIGN EQUATION/PARAMETER
w/l megohm (w/l megohm)**
__________________________________________________________________________
1) e(signal) = I(in) .times. R(feedback)
1 V = 2 ua .times. 1 meg
1 V = 0.5 ua .times. 4 meg
[terminated at output] (.25 V = .5 ua .times. 1 meg)**
2) R(in) = 1/2pi .times. f .times. c
796 ohm = R(in)
3184 ohm = R(in)
(796 ohm = R(in))**
3) A(loop) = R(feedback)/R(in)
1256 = 1 meg/796 ohm
1256 = 4 meg/3148 ohm
(1256 = 1 meg/796 ohm)**
4) e(noise) = (nv/(H 0.5)) .times. A(loop)
e(noise) = 6 mv
e(noise) = 6 mv
(e(noise) = 6 mv)**
5) e(out) = e(signal) + e(noise)
e(out) = 1 v + 6 mv
e(out) = 1 v + 6 mv
(e(out) = 1 v + 6 mv)**
6) Z(transfer) = e(out)/I(in)
Z(trans) = 1 v/2 ua
Z(trans) = 1 v/0.5 ua
(Z(trans) = .25 v/.5 ua)**
7) signal to noise - preamp only
45 db 45 db
(45 db)**
8) frequency response of input
20 mhz 5 mhz
(20 mhz)**
9) frequency response of preamp
20 mhz 20 mhz
(20 mhz)**
10)
signal to noise - system overall
45 db 63 db
with post bandwidth filter (51 db)**
__________________________________________________________________________
**indicates prior art case
In operation, the operator selects either a fluoroscopic mode or a
radiographic mode on the console keyboard. When the fluoroscopic mode is
selected, the switch logic unit 46 operates switches 78 and 82 to connect
the 1 megohm feedback resistor path 74 and the 20 MHz filter path 88. When
the radiographic or 7.5 frames per second mode is selected, the switch
logic unit 46 operates switches 78 and 82 to connect the 4 megohm feedback
resistor path 76 and the 5 MHz bandpass filter 84. Selecting the 4 megohm
feedback resistor over the 1 megohm feedback resistor and the 5 MHz filter
over the 20 MHz path effects a 12 db noise improvement. It is to be
appreciated that it is possible, using the apparatus and method of the
present invention to provide a manner in which the selected feedback may
be changed without changing the bandwidth or in the alternative changing
the bandwidth without changing the feedback.
The invention has been described with reference to a preferred embodiment.
Obviously, modifications and alterations will occur to others of ordinary
skill in the art upon reading and understanding the preceding
specification. It is intended that the invention be construed as including
all such alterations and modifications insofar as they come within the
scope of the appended claims or the equivalents thereof.
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